Crankcase breather aka positive crankcase ventilation (PCV). You can either put a filter on it or run a hose to the intake MAF tube. The factory system has a check valve and a hose that runs to the intake side.
It's plugged now and I assume that's so you don't get anything in it, but you will want to make darn sure you unplug it before you run the car. The buildup of pressure can blow out your seals and cause other issues.
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